cakpenter: development of the oculomotoe nerve. ]61 



stage K, the third nerve arises from the ventral face of the mid-hrain 

 by a root triangular in shape, containing many " nerve cells." Eunning 

 caudad and laterad, the nerve reaches the interval between the dorsal 

 ends of the first and second head cavities, vrhere it expands into a small 

 ganglion. At this ganglion, the nerve divides into two main branches, 

 one running cephalad along the top of the first head cavity to the 

 extreme anterior end of the head, the other passing ventrad between 

 the first and second cavities. A short branch, coming directly from the 

 Gasserian ganglion, enters this small ganglion and later, uniting with 

 the first branch of the' oculomotor described above, forms the ramus 

 ophthalmicus profundus of the adult ; tliis ramus has, in most cases, 

 the appearance of being a branch of the trigeminal nerve. From the 

 posterior wall of the first head cavity are derived those muscles of the 

 eyeball which later are innervated by the oculomotor. The author 

 considers this small ganglion between the tops of the first two head 

 cavities to be the ciliary ganglion of the adult, and agrees with Schwalbe 

 that it belongs exclusively to the third nerve. 



It is evident from later investigations by others that Marshall is here 

 dealing with the mesocephalic ganglion, and that his first branch of the 

 oculomotor has only an apparent connection with that ilerve, being, as a 

 matter of fact, the ramus ophthalmicus profundus of the trigeminus, 

 with which the mesocephalic ganglion is primitively connected. The 

 close contact into which the two nerves come in the region of the 

 mesocephalic ganglion accounts for the author's failure to separate them. 

 (Comp. van Wijhe, '82.) 



The sixth or abducent nerve was first observed in stage 0, some time 

 after the oculomotor had made its appearance. It springs from the 

 ventral face of the hind-brain by a large number of slender roots, and 

 runs to the fundament of the posterior rectus muscle. The roots as 

 well as the trunk of the nerve contain many more or less elongated, fusi- 

 form cells, but none of these are ganglion cells. 



Marshall and Spencer ('81) confirm Marshall ('8l) without adding 

 anything of importance to the latter's account. 



In 1882 van Wijhe published his excellent description of the develop- 

 ment of cranial nerves in selachians. In Balfour's stage I he found 

 the fundament of a ganglion connected with the ophthalmic branch of the 

 trigeminus, and representing the anterior extremity of the neural crest. 

 This ganglion — called the ciliary by van Wijhe, but evidently the 

 mesocephalic of our nomenclature — at first lies immediately under the 

 epidermis, but soon moves away from it in the direction of the gan- 



VOL. XLVIII. — NO. 2 11 



